1. Field of the Invention
This invention concerns a flexible joint for a riser connecting a structure located on the seabed and a surface installation such as a ship or articulated column head, to facilitate exploration and exploitation of hydrocarbon deposits, or for any similar purpose.
Such joints, which have to allow the continuity of the passage formed by the riser pipe, also must permit deviation of at least 10.degree. in all directions, as well as withstanding considerable stresses in all positions, whether of tractional, flexional or torsional nature.
Experience shows that means of making jointed or swivel couplings watertight must be different from the devices intended to make the joint flexible and at the same time give it a certain rigidity.
2. Description of the Prior Art
Various types of joints exist in the prior art, in which resilient components are combined with a swivel coupling.
U.S. Pat. No. 3,433,504 describes a joint in which an elastomer toric ring is placed between a bearing surface forming part of a male component, and a seating surface forming part of a female component of a swivel coupling, thereby providing a clear distinction between the two functions: transmission of longitudinal stresses, and positioning in various angular orientations. However, such a system is suitable only for small deviations of a few degrees, and in installations intended to permit deviation of more than 10.degree. several connectors, each containing a torus ring, must be superimposed, which makes such systems cumbersome and complicated to operate.
U.S. Pat. No. 4,076,284 describes a system in which semispherical resilient pads and fretted semispherical pads are placed between the two homologous parts of a swivel joint. In other systems, such as described in U.S. Pat. No. 4,068,868 for example, these fretted elastomer pads are placed between semispherical bearings on the male and female components of the swivel joint and an intermediate component, or core, of the joint.
Such devices permit extensive deviation in all directions, with the necessary amount of rigidity. However, although such semispherical fretted pads can withstand compressive forces satisfactorily while the riser pipe is vertical, shearing stresses are set up as soon as it tilts. Therefore since the modulus of compression E is known to be some 5 times greater for such materials than the shearing modulus G, parts which have to withstand shearing stresses have to be much larger than those subject only to compressive stress.
The present invention diminishes these difficulties and eliminates the drawbacks associated with placing the fretted components between concentric spherical parts while keeping them between thrust-plates which remain parallel regardless of the relative movement of the two parts of the riser pipe.